I would estimate it to be at least half of the blood used in the movie, which was hundreds of gallons according to the trivia.
Which brings up a question. Another post was made about what they would have put as the cause of death. I wonder what they thought of finding way more blood than would be in one human body, and did they lead them to investigate the possibility of many victims being in that room?
Top Ten Classic Horror Movie Working Titles -
http://landowonder.com/top-ten.html

Except when he became 40 gallons of blood.
Green Goblin is great!